520 Transactions. — Miscellaneoiis. 



the elements they require for the support of their tiny bodies. 

 The other elements of the organic compounds are thus let 

 loose, and are seized by the oxygen of the air between the 

 particles of sand, forming the mineral compounds already re- 

 ferred to as nitrates and nitrites. 



The presence of oxygen in the air-spaces of the sand is 

 thus essential to the chemical change ; but the constant 

 saturation in the first experiment described prevents the re- 

 newal of the air-supply, and the success of the purification 

 process is thus frustrated. Hence the necessity for intermit- 

 tency in the application of the liquid to be purified. When 

 the fiow is stopped the contained fluid drains off, and air, 

 bearing its bounteous supply of oxygen, again fills the spaces. 

 The new compounds formed by this chemical process (oxida- 

 tion or nitrification, as it is called) are the nitrates and nitrites, 

 which, being soluble, are carried on with the water, providing 

 there are no plants to feed vipon them. But nitrates consti- 

 tute the best and most easily-assimilated food for plants ; and 

 as the chemico-biological process I have described takes place 

 most largely near the surface, where the bacteria are most 

 numerous, it is apparent that the most favourable conditions 

 for the growth of plants are here to be found. The assimila- 

 tion of the nitrates and nitrites by the plants in the above 

 experiment, illustrative of broad irrigation, explains their 

 absence from the effluent water. 



These principles applied to the disposal of sewage by appli- 

 cation to the soil constitute the scientific basis of sewage farm- 

 ing. But a sewage farm, to be successful, must effectually dis- 

 pose of sewage as sewage by converting the solids into plants, 

 and the liquids into pure water, without nuisance or injury to 

 health. Now, can this be done ? The experience of years rings 

 out a chorus of emphatic affirmation. If the laws arising out of 

 chemico-biological experiments be conformed to in the manage- 

 ment of a sewage farm the solids can be converted into the 

 finest plants and the liquids into the purest water without 

 the slightest nuisance or injury to health. Dr. Cornill tells 

 us that he drank freely of the limpid streamlets of the Berlin 

 Sewage Farm. Many sewage farms are favourite Sunday 

 resorts, and strangers are frequently surprised to be told, while 

 they admire the luxuriant vegetation, and breathe the pure 

 air, that they are on one of the walks in the middle of a 

 sewage farm. 



But though this brilliant consummation can be attained, 

 and is being attained in some of the most populous towns in 

 different parts of the world, it is not an easy nor an inexpen- 

 sive matter. Conditions must be favourable to the working 

 of a sewage farm. Land must be available at an elevation 

 sufficient to permit of underdraiuage if the soil is retentive of 



